1. Field of the Invention
The present invention relates to image-forming apparatuses, such as copy machines and printers, using recording techniques such as electrophotographic recording and electrostatic recording and recording-medium-temperature detector unit used in the image-forming apparatuses. More specifically, the present invention relates to an image-forming apparatus having a temperature detecting portion for detecting a temperature of a recording medium after a heat-fixing process and a recording-medium-temperature detector unit used in the image-forming apparatus.
2. Description of the Related Art
A typical image-forming apparatus, such as a copy machine and a printer, using recording techniques such as electrophotographic recording and electrostatic recording includes a fixing device for fixing a toner image formed on a recording medium by applying heat, and various techniques for improving the fixability of the image are suggested.
For example, a method in which the temperature of a recording medium is detected after a heat-fixing process and feedback control is performed for obtaining a desired temperature in a fixing device has been suggested (refer to, for example, Japanese Patent Laid-Open No. 1-150185, Japanese Utility Model Laid-Open No. 1-160473, Japanese Patent Laid-Open No. 3-53276, Japanese Patent Laid-Open No. 4-181250, Japanese Patent Laid-Open No. 6-308854, Japanese Patent Laid-Open No. 7-230231, Japanese Patent Laid-Open No. 7-239647, Japanese Patent Laid-Open No. 10-161468, Japanese Patent Laid-Open No. 2000-66461, Japanese Patent Laid-Open No. 2001-13816, Japanese Patent Laid-Open No. 2002-23555, Japanese Patent Laid-Open No. 2002-214961, and Japanese Patent Laid-Open No. 2003-29485).
FIG. 10 shows an example of a heat-fixing device in which the recording medium temperature is detected by a non-contact sensor after a heat-fixing process. In this heat-fixing device, a non-contact sensor 20, such as an infrared radiation sensor, is positioned downstream of a fixing nip portion for measuring the recording medium temperature without contact.
FIG. 11 shows an example of a heat-fixing device in which the recording medium temperature is detected by a contact sensor after a heat-fixing process. In this heat-fixing device, a temperature sensor 18, such as a thermistor, is positioned downstream of a fixing nip portion and an opposing member 19, such as a rubber roller, is positioned so as to face the temperature sensor 18. The temperature of the recording medium is measured while the recording medium is nipped between the temperature sensor 18 and the opposing member 19.
However, in the case in which the recording medium temperature is detected and used in the feedback control, there is a problem that the temperature cannot be detected with sufficient accuracy.
In the heat-fixing process of the recording medium, moisture in the recording medium is also heated, and water vapor is emitted from the surface of the recording medium. When a non-contact sensor is used for temperature detection, it is difficult to accurately detect the recording medium temperature since the water vapor adheres on the surface of the non-contact sensor.
In addition, in the case in which the temperature sensor is brought into contact with the opposing member like the roller and the recording medium temperature is detected while the recording medium is nipped between the temperature sensor and the opposing member, the heat of the recording medium is dissipated into the opposing member. Therefore, it is also difficult to accurately detect the recording medium temperature in this case.
On the other hand, the size of image-forming apparatuses has recently been reduced, and it is difficult to provide a space for an additional temperature sensor.